The present invention relates to electrically programmable integrated circuits. Specifically, a programmable integrated circuit is provided having an array of anti-fuse links which can be activated by applying a higher than usual circuit voltage to an individual anti-fuse link.
Programmable links are used extensively to form programmable read only memory devices. These devices may be user programmed to provide data which is to be used for various circuit trimming functions. Analog type components such as comparators, amplifiers, etc. made in MOS technology may require some parameter adjustments, and fusible links may be used extensively in order to provide a selective trimming of the parameter values. Currently, MOS devices may use up to 100 such fusible links on a single integrated chip. Each fusible link may be individually addressed, and blown using a high intensity laser beam. The blown fusible link interrupts a current path whose change of state from conducting to non-conducting represents a logic state.
A limitation on using such fusible links includes the large amount of power dissipation necessary to blow them. The necessary power cannot be achieved in smaller sized fusible links, thus inhibiting a reduction of their size comparable to the reduction in size of other circuit components used in an integrated circuit.
The anti-fuse device is similar, however it is blown by applying a higher than normal voltage to the anti-fuse, which produces a short circuit where an open circuit once existed. This change in state also represents a change in logic state which may be used to store data.
In order to keep the pinout low on integrated circuit packages which utilize the anti-fuses, addressing schemes are provided so that individual anti-fuses may be addressed with a small number of external pin connections to the integrated circuit chip.
Future increases in circuit density and the corresponding increase in the number of devices on an integrated circuit will increase the need for more anti-fuse devices for circuit trimming functions. Whereas presently such circuit devices may include up to 100 fusible links, the need for additional fusible links, in the 300 to 500 link range is becoming necessary.
The anti-fuse device generally comprises two conductors, either metal and/or a semiconductor material having some kind of dielectric or insulating material between the two conductors. In the recent past this dielectric was set to approximately half the normal thickness of a FET thin oxide gate, so that in the presence of a high voltage, it was electrically broken down to change from a non-conducting state to a conducting state, while not affecting in any permanent way the remaining components which have thicknesses two or more times that of the dimension between the conductors of an anti-fuse.
As integrated circuit density keeps increasing, it is becoming more difficult to reduce the thickness of the anti-fuse insulator. For instance, gate insulator thickness is now approaching 50 .ANG.. Manufacturing transistors with insulator thicknesses half this distance can result in other deleterious effects occurring, such as tunnelling between conductors which would then be spaced only 25 .ANG. apart.
The physical size limitations represented by these higher densities on the smaller anti-fuse devices makes it impossible to maintain a high voltage differential between the fusing potential of the anti-fuse device and that of other integrated circuit devices which are not intended to be fused. For these reasons, it is desirable for higher density integrated circuit packages to provide adequate isolation between the higher voltages applied to fuse the anti-fuse link and the remaining components in the circuit.